1. Field of the Invention
The present invention relates to a plasma display panel and a method of controlling the brightness of the plasma display panel used for a TV display, an advertisement indicator and so on.
2. Description of the Prior Art
Conventionally, the expression of gradation on a plasma display is achieved by introducing several sub-fields for weighting the brightness level in a field for a 1/60 second period as shown in FIG. 16(a). In FIG. 16(a), there are 8 sub-fields, sub1, sub2, . . . sub8, each of which consists of a writing period for data writing discharge, a maintaining period for luminescence display discharge and a erasing period for terminating the maintained discharge. The length of the maintaining period of each sub-field is different according to the weighting brightness level of each sub-field.
For example, each brightness of sub1, sub2, . . . sub8 is weighted as 2.sup.0 *B.sub.0, 2.sup.1 *B.sub.0, . . . , 2.sup.7 *B.sub.0 (B.sub.0 : Unit brightness), and a 256 level gradation with linear characteristic can be controlled by the combination of above mentioned weighted sub-fields in a field. If the brightness weighting levels are multiplied, for example, as A*2.sup.0 *B.sub.0, A*2.sup.1 *B.sub.0, . . . , A*2.sup.7 *B.sub.0 (A&gt;1), a 256 level gradation with linear characteristic which is A times the brightness of the standard level can be controlled by the combination of above mentioned multiplied weighted sub-fields in a field as shown in FIG. 16(b). Comparing FIG. 16(a) and FIG. 16(b), the lengths of all maintaining periods of FIG. 16(b) are A times longer than those of FIG. 16(a) because the weighting level is A times larger.
A TV signal has 1/2.2 .gamma. characteristic. Therefore, the plasma display panel should apply the reverse .gamma. correction to the display data which has the above-mentioned linear gradation display characteristic.
The block diagram of the conventional basic configuration for controlling the brightness of the plasma display is shown in FIG. 17. As shown in FIG. 17, the input signal is once converted to a 256 gradation weighted brightness level which has a linear characteristic by an 8 bit gradation display processing unit 31 whose maximum brightness is M.sub.1, then converted to the output signal for the right brightness display with maximum brightness M.sub.1 via a reverse .gamma. correction unit 32 for the .gamma.=2.2 correction.
In order to explain the above-mentioned data conversion in detail, the relationship between the TV input signal level and the display brightness is shown in FIG. 18. As shown in FIG. 18, the temporal relationship between the input signal level and the display brightness is shown as the chain line 1, whose characteristic is given by .gamma.=1 by the 8 bit gradation display processing unit 31 shown in FIG. 17, then the relationship is corrected as the solid line 2 which characteristic is given by .gamma.=2.2 correction by the reverse .gamma. correction unit 32. Consequently, the plasma display panel can display the data up to the maximum input signal level LM as the right brightness up to the maximum brightness M.sub.1. In the same manner, if the brightness weighting levels are multiplied by A times as shown in FIG. 16(b), the temporal relationship between the input signal level and the display brightness is shown as the chain line 3 in FIG. 18, whose characteristic is given by .gamma.=1 by the 8 bit gradation display processing unit 31 shown in FIG. 17. Then the relationship is corrected as the solid line 4 whose characteristic is given by .gamma.=2.2 correction by the reverse .gamma. correction unit 32. Consequently, the plasma display panel can display the data up to the maximum input signal level LM as the right brightness up to the maximum brightness M.sub.2, which is A times larger than M.sub.1.
As one embodiment of the conventional plasma display panel, a block diagram of a plasma display driving circuit and panel that achieves the above-mentioned brightness control is shown in FIG. 19. A plasma display panel unit 100 comprises an AC type plasma display panel 101 which has M column data electrodes D.sub.1, D.sub.2, . . . , D.sub.M and N line pairs of the scanning electrodes and the maintaining electrodes SC.sub.1.SU.sub.1, . . . , SC.sub.N.SU.sub.N in a N*M matrix status, a data driver 102 for driving the M column data electrodes, and a scanning.maintaining.erasing driver 103 for driving the N line pairs of the scanning electrodes and the maintaining electrodes.
Next, the flow of the signal for driving the plasma display panel unit 100 is explained as follows. The level of an input signal is adjusted by a level adjusting unit 11 and inputted to an 8-bit A/D converter 12. The outputs of the A/D converter 12 are inputted to the reverse .gamma. correction unit 13 for the .gamma.=2.2 correction. After the .gamma. correction, the data are stored in the frame memory 14, and outputted therefrom via the output processing unit 15 to drive the data driver 102. On the other hand, the timing pulse generator 17 is driven by the input signals via the synchronous separator 16. In addition, the output signals of the timing pulse generator 17 control the A/D converter 12 as well as a memory controller 18 and a driving timing generator 19. This timing pulse generator 17 sets the timing for the writing period, the maintaining period and the erasing period as well as the timing for deciding the length of the maintaining periods according to the brightness weighting level of each sub-field. The output signals of this driving timing generator 19 drive the scanning.maintaining.erasing driver 103 and feedback to the memory controller 18. The memory controller 18 controls the read-out and the read-in of the frame memory 14 in order to drive the data driver 102 via the output processing unit 15 by synchronizing to the output signals of the timing pulse generator 17 and the driving timing generator 19.
Consequently, the relationship between the TV input signal level and the brightness display of the plasma display panel 101 becomes the solid curved line 2 shown in FIG. 18 processed by the circuit and the maximum brightness M.sub.1 is outputted corresponding to the maximum input signal level LM. If the brightness weighting levels of the sub-fields are multiplied by A times for prolonging the length of the maintaining period by the driving timing generator 19, the relationship becomes the solid curved line 4 shown in FIG. 18 and the maximum brightness M.sub.2 that is A times larger than M.sub.1 is outputted corresponding to the maximum input signal level LM.
The above-mentioned conventional plasma display panel and method of controlling the brightness of the plasma display panel has the following problem.
FIG. 20 shows an example of an input signal for TV broadcasting. In order to display all variations of the input signal as the variation of the brightness on the plasma display panel, the signal level should be set as the peak input signal level LP and the constant of the brightness weighting value A and the value of .gamma. should be adjusted for setting the maximum brightness level be M.sub.2 shown in FIG. 18. By this setting and adjusting, the relationship between the input signal and the brightness of the plasma display panel has a linear characteristic as shown in FIG. 21. Consequently, the waveform of the brightness on the plasma display panel will be the same waveform of the corresponding input signal correctly shown as FIG. 22. Therefore, correct gradation display up to the peak brightness corresponding to the peak input signal level LP can be achieved. However, since the mean value of the input signal of the TV broadcasting is normally 20% to 30% of the peak input signal level LP, the mean value of the brightness will deteriorate. That is, the pictures on the plasma display will become dark as a whole.
This brightness deterioration problem seems to be solved by setting the constant of the brightness weighting value A and the value of .gamma. sufficiently large to enhance the mean value of the brightness. However, this enhancement will bring another serious problem in that the temperature rise of the plasma display panel becomes large and the temperature will be above the permissible temperature due to the augmentation of the electric power consumption. In order to keep the temperature of the plasma display panel within the permissible temperature, the maximum brightness level should be about 420 cd/m.sup.2 at most. In this case, the mean value of the brightness on the plasma display panel corresponding to the TV broadcasting input signals will be at a low level of 80 cd/m.sup.2 to 120 cd/m.sup.2. When setting the input signal level to the A/D converter 12 in FIG. 19 to be up to the maximum input signal level LM (detection signal level LD) shown in FIG. 20 by means of the level adjusting unit 11, the relationship between the input signal level and the brightness display on the panel becomes the line shown in FIG. 23. In this case, the gradation display can be applied to the input signals up to the maximum input level LM, and the brightness applied to the input signals which surpass the maximum input level LM becomes the constant saturated maximum brightness shown as FIG. 24. Therefore, both the maximum brightness and the peak brightness become 420 cd/m.sup.2, and there is no gradation between the brightness corresponding to the maximum input signal level LM and the brightness corresponding to the peak input signal level LP. This problem deteriorates the quality of the display on the plasma display panel.
Consequently, above-mentioned conventional plasma display panel and method of controlling the brightness of the plasma display have difficulty in achieving higher brightness display due to the limitation of the maximum brightness level in order to satisfy the maximum permissible power of the plasma display panel.